1. Field of the Invention
This invention is directed to the treatment of aqueous systems, and more specifically to inhibiting scale formation and other solid deposits in industrial heating and cooling systems.
2. Description of the Related Art
The water used in industrial systems such as in steam generating boilers, hot water heaters, heat exchangers, cooling towers, pipelines, gas scrubbing systems and related equipment accumulate various impurities derived from the water. These impurities generally include the alkaline earth cations, such as calcium, barium and magnesium, and some bicarbonates and carbonates, sulphates, phosphates, silicates and the like. The most common impurities in industrial water are the water-hardening metal ions including calcium, magnesium and the carbonate ions. In addition to precipitating as carbonates, calcium and magnesium, as well as the other metals such as iron or copper, react with the sulphates or phosphates to form the respective insoluble complex salts. These reaction products accumulate on the surfaces of the system forming scale and sludge, which substantially reduce the heat transfer efficiency by settling in the systems where they impede flow and insulate heat-transfer surfaces. Moreover, in addition to interfering with the fluid flow and heat transfer, corrosion of the metal surfaces is promoted, since the corrosion inhibitors added to the water are less able to reach the metal surfaces to provide effective protection against the corrosive components within the aqueous system. Further, scale deposits can harbor bacteria, the removal of which is expensive due to the delays inherent in the shutdown-treatment-restart sequence. Maintaining proper residual levels of water treatment chemical actives is critical to the success of high-performance water treatment programs. To provide optimum cost and performance, each active component in the water treatment program must be consistently maintained at residual levels sufficient to achieve treatment efficacy without relying on unnecessarily high levels of treatment chemicals.
Water conservation is another important consideration in the operation of industrial cooling and heating systems and will become more important in some regions as a result of moderate to severe water shortages caused by climate change, contamination and/or population growth. In the face of such shortages, the cost of both water consumption and discharge for industrial users will continue to increase. Additionally, both increased governmental regulation and a desire to reduce costs have motivated industrial users, particularly in industries that have traditionally utilized high levels of water consumption, to identify and apply methods that will enable them to reduce the incremental water consumption.
Improvements in water treatment technology, by allowing increased use of recycled water and permitting increased cycles of operation, have been significant factors in reducing industrial water consumption and discharge, ideally without requiring extensive process redesign or capital investment. However, both the use of recycled water and the use of higher operating cycles generally increase the potential for fouling and place correspondingly greater demands on the water treatment programs.
Early water treatment programs, particularly those for evaporative cooling systems, utilized acid to control the pH and thereby reduce the potential for scaling. More recently, however, the development and increasing use of acid-free organic cooling water treatment programs, also referred to as all-organic programs (AOP), has shifted the focus of high-cycle operation towards eliminating or controlling the deposition of calcium carbonate, calcium phosphate, and magnesium silicate scale. In addition to the scaling concerns, when using higher cycles of concentration in cooling systems, other makeup water components, including iron, ammonia, and the total dissolved solids (TDS), can place severe demands on the treatment program chemicals to control the resulting corrosivity and conductivity of the aqueous system. A treatment program that fails to address these additional concerns can result in galvanic corrosion, interfere with inhibitor film formation, and/or reduce the effectiveness of certain biocides.
The calcium carbonate deposition potential of a cooling water is frequently expressed as a scaling or saturation index. One such index is the Langelier saturation index (LSI) which provides an indication of calcium carbonate (CaCO3) stability in an aqueous system. The LSI is a function of the calcium hardness, alkalinity, conductivity and temperature of the aqueous system. A typical AOP can operate satisfactorily in aqueous systems having an LSI between 1.0 and 3.0, but few, if any, are able to function satisfactorily at LSI values above 3.0.
However, LSI calculations are based on bulk water concentrations of calcium and alkalinity and do not take into account other soluble species that may effect the activity of the calcium or carbonate ions. For this reason, the inventors prefer to use the calcite saturation index (CSI) for evaluating treatment program performance. The CSI defines the relative degree of saturation of calcium carbonate as a ratio of the ion activity product to the solubility product according to the formula:
CSI=[Ca2+][CO32]/Ksp CaCO3
Unlike the LSI method, the CSI calculation takes into account the effects of ion pairing and can be used to compare the scaling tendency of waters of with very different compositions. Commercially available software applications, such as Water Cycle(trademark) from French Creek Software, Kimberton, Pa., permit rapid calculation of CSI and many other water parameters based on makeup water chemistry.
Most AOPs can achieve satisfactory results in aqueous systems having an operating CSI of between 100 to 200. Although there have been reports of an AOP that that can function satisfactorily in an aqueous system having an operating CSI of approximately 300, in practice such supersaturated waters generally present an unacceptable risk of total bulk water precipitation. Bulk water precipitation can be a catastrophic event in an industrial system resulting in, at a minimum, extensive fouling of the system, and, at worst, actual structural failure. Any practical treatment system must, therefore, provide a sufficient operating margin to avoid such an occurrence.
In addition to calcium carbonate scale, industrial systems must generally contend with silicates as well. Silicates may deposit on heat transfer surfaces as a scale of colloidal silica or as magnesium silicate and may, in some instances, become a limiting factor in a given aqueous system. In particular, the scaling potential of magnesium silicate increases for values of the system pH above 8, but does not typically become an significant concern until the system pH exceeds 8.5. Silica can co-precipitate with iron and magnesium hydroxides and the silicate may also precipitate with calcium salts. In systems with an alkaline pH, levels of silicate in general should be kept such that the product of the magnesium and silicon concentrations (in parts per million) is below 20,000 (i.e., Mg*Si less than 20,000). It is especially important to avoid the formation of silicates because, once formed, silicate deposits are particularly difficult to remove. To address this problem, various polymeric materials have been developed which show an ability to inhibit colloidal silica and magnesium silicate deposits. These materials are, however, relatively expensive and are thus usually restricted to specific applications where silicate is the dominant contaminate and the potential cost savings justify their use.
Increased phosphate levels in the available makeup water is also becoming an issue for many industrial applications that are attempting to maintain high-cycle operation. Phosphate may be present in surface waters as a result of agricultural run-off or industrial pollution. The concentration of phosphate in surface waters may also vary over a wide range due to various seasonal and/or drought conditions. Further, municipal water supplies may contribute to the phosphorus content of industrial water by adding polyphosphates as a corrosion inhibitor to protect distribution lines. The addition of polyphosphates is also used in some municipal water supplies to reduce the appearance of xe2x80x9cred waterxe2x80x9d due to the presence of iron oxides. In both of these situations, the treatment levels in municipal waters are typically around 1 mg/liter of polyphosphates. Corrosion inhibition can also be improved with the addition of a small amount of zinc. Reversion of polyphosphates to phosphate occurs under low pH conditions, high temperature, and/or in the presence of metal oxides, such as iron oxide. The combined effect of all of these various sources of phosphate in the available makeup water can lead to significant calcium phosphate deposition problems, particularly in systems having an alkaline pH and operating at higher cycles of concentration.
Particularly at high operating cycles, it is important to maintain treatment chemical levels above certain minimum operating levels. Given the high degree of saturation that will typically characterize high-cycle aqueous systems, failing to maintain sufficient levels of the active inhibiting components can quickly result in significant scale formation. Although maintaining the treatment chemical concentrations at levels significantly above the specified minimum level may provide an increased safety margin, this technique is generally undesirable for several reasons. For example, maintaining excessively high levels of treatment chemicals represents both an unnecessary, and possibly significant, expense. Further, many of the most common treatment chemicals have solubility limits that will determine the maximum safety margin that can be established and maintained. Additionally, some treatment chemicals become corrosive at the high concentration levels that would be necessary to establish the desired safety margin.
While measurement of product tracers has proven to be a convenient way to estimate product concentration, methods for measuring the actual concentration of individual treatment actives are gaining increased attention. This is particularly important in higher stress conditions where individual components may be lost or consumed at dramatically different rates. Recent publications have shown the applicability of measuring specific polymeric dispersants through a simple field test. One such method is described U.S. Pat. No. 6,153,110 to Richardson et al., (xe2x80x9cRichardsonxe2x80x9d) the contents of which is incorporated herein by reference. Richardson discloses a technique for determining both a quantitative measurement of the polymer component and differentiating between Free and Total polymer in the aqueous system to give greater insight into dispersant efficiency and the potential onset of scaling.
The majority of acid free AOP cooling water treatment programs are based on combination of phosphonate scale inhibitors and polymeric dispersants along with yellow metal corrosion inhibitors. At high cycles of operation, the choice of polymeric dispersant is key to the overall performance of the treatment program. The majority of commonly used polymers have certain known strengths, but are also known to suffer from some distinct performance weaknesses, particularly when employed in high-cycle, high-stress cooling water applications. For example, polyacrylates are generally effective calcium carbonate inhibitors and dispersants, but under high hardness conditions they may precipitate as calcium salts. Polyacrylates are also sensitive to the presence of iron and are only weak calcium phosphate inhibitors. Polymaleates are strong crystal modifiers which, when used in high-cycle applications, give excellent scale control, particularly when used in combination with phosphonates, but are weak calcium phosphate inhibitors and relatively poor dispersants. Sulfonated co-polymers and ter-polymers are excellent calcium phosphate inhibitors, can tolerate iron, and are generally good dispersants but are, however, inferior to polyacrylates and polymaleates as calcium carbonate inhibitors.
This invention relates to a novel composition and a method for treating aqueous systems to prevent the accumulation of mineral scale and corrosion which comprises adding a treatment solution comprising a phosphonate (2-phosphonobutane-1,2,4-tricarboxylic acid), and a quad-polymer, Quadrasperse(trademark), that includes four discrete monomers (specifically allyloxybenzenesulfonic acid (about 3.5 mole %), methallyl sulfonic acid (about 2.5 mole %), a copolymerizable non-ionic monomer (about 13-18 mole % methyl methacrylate) and an olefinically unsaturated carboxylic acid monomer (about 76-81 mole % acrylic acid)) and has an average molecular weight of less than about 50,000, to the aqueous system. The quad-polymer composition according to the present invention overcomes many of the limitations of conventional polymers now used in water treatment. In particular, the quad-polymer composition provides stronger calcium carbonate inhibition than known co-polymers or ter-polymers, lacks the calcium sensitivity issues that may exist with polyacrylates, and provides better dispersancy than polymaleates.
It is an object of the invention to provide a water treatment composition that provides excellent calcium phosphate and calcium phosphonate inhibition and dispersancy.
It is a further object of the invention to provide a water treatment composition that exhibits calcium carbonate deposit control at CSI/LSI limits exceeding those possible with traditional AOPs.
It is a further object of the invention to provide a water treatment composition that exhibits superior thermal stability at high temperatures.
It is a further object of the invention to provide a water treatment composition that exhibits good calcium carbonate inhibition and iron dispersancy.
It is a further object of the invention to provide a water treatment composition that exhibits superior magnesium silicate deposit control.
It is a further object of the invention to provide a water treatment composition that is directly measurable by field test methods.